Just recently I've had to implement camera preview on Android. This common task has required some knowledge of how transform matrices work. For me personally maths has been indispensable during my career.
What do you mean by "instead"? It seems to me they support both languages as equals (the Android documentation contains examples in both languages) and have no plans of ditching Java.
I pretty much stopped freelancing once oDesk and Elance merged into Upwork, but back then I could probably earn between 10k and 20k USD annually before taxes. Admittedly, I'm a lazy programmer. Good luck with your project!
I personally quite liked Java: The Complete Reference by Herbert Schildt – as the name suggests, it covers most of the topics relevant to Java developers, including some outdated ones.
Head First Java by Kathy Sierra is also a decent introduction to the language, though much more limited and outdated.
Oracle also has on-line Java tutorial which I regularly visit during development. However the content is more scattered than in a book.